Cutting tools comprising a plurality of cutting blades projecting axially from the face or periphery of a circular cutter head are well known for producing bevel ring gears and pinions by face milling or face hobbing processes. Such tools and/or cutting blades can be seen, for example, in U.S. Pat. No. 4,575,285 to Blakesley; U.S. Pat. No. 3,192,604 to Whitmore; or, U.S. Pat. No. 4,530,623 to Kotthaus. Cutting blades such as those discussed in these patents are usually formed of high speed steel but may also comprise carbide material composites (e.g. cemented carbides) formed by powder metallurgy processes. Such composites comprising, for example, tungsten carbide (WC) in a cobalt (Co) matrix.
When it becomes necessary to sharpen a cutting blade, one or more surfaces of the cutting blade are usually ground to restore the cutting edge to an acceptable cutting condition. For example, in the stick-type cutting blades disclosed by U.S. Pat. No. 4,575,285, only the cutting side and clearance side flank surfaces need to be ground to sharpen the cutting edge. Sharpening cutting blades of this type may be accomplished in several known ways among which are the methods disclosed in U.S. Pat. No. 4,170,091 to Ellwanger et al. or U.S. Pat. No. 5,168,661 to Pedersen et al. In U.S. Pat. No. 4,503,623, which also discloses sticktype cutting blades, the side surfaces are likewise ground but it is also necessary to grind the front surface in order to sharpen the cutting blades.
The cutting blades described in U.S. Pat. No. 3,192,604 are of the known form-relieved type and these cutting blades are sharpened by grinding of the front face only. An example of a process for sharpening form-relieved cutting blades can be found in U.S. Pat. No. 5,503,588 to Sweet.
Subsequent to sharpening of many cutting blades, especially those comprising brittle materials such as carbides, it is appropriate, or even necessary, to treat the cutting edge in some manner to prevent it from chipping at the beginning of the cutting operation. In carbide materials, one reason for chipping is that carbide/metal-matrix composite is very brittle and a very sharp cutting edge presents a problem in that, at the thin cutting edge, the particles of carbide (e.g. WC) and/or matrix material (e.g. Co) are not supported sufficiently. The cutting process, especially at the beginning of the cutting process, tends to break away some of the inadequately supported particles, particularly carbide particles, from the cutting edge resulting in pockets formed in the cutting edge thus marring the surface being machined.
Previous efforts to address the breaking-away condition of a cutting edge have been directed to forming a defined radius along the cutting edge. The radius, in comparison to a sharper edge, leads to a more stable cutting operation and uniform wear. Carbide particles along the cutting edge are exposed to significantly less chipping. Some of the methods of rounding cutting edges are sand blasting with silicon carbide, drum rotation with abrasive particles, hand filing using a soft steel pipe, and grinding or polishing with brushes.
It is known, for example, to treat carbide cutting inserts with a rotating brush approaching the cutting edge at an angle of between 45 degrees and 90 degrees and moving along the entire width of the cutting edge. The rotating brush comprises nylon bristles having a cross-section of one square millimeter (1 mm.sup.2) and having 120 grid silicon carbide incorporated into the nylon bristles. The surface speed of the rotating brush in the edge treating operation is 50 feet per second (15 meters/second).
However, in the case of cutting tools for toothed articles, and in particular stick-type carbide cutting blades for cutting bevel and hypoid gears, the above methods have proven to be unsuccessful. For example, a bevel gear stick-type cutting blade has a cross section of up to 3/4 inch.times.3/4 inch (19 mm.times.19 mm) and may have a cutting edge ground onto it with a length of 1 inch (25.4 mm). The cutting edge is formed by the intersection of the cutting side flank surface, which is relieved at an angle of, for example, 6 degrees, and the front face which is oriented at a particular rake angle usually in the range of -20 degrees to +20 degrees.
The distribution of the dimensions and shape of the bevel cutting stick-type blade is not comparable to any other existing carbide tools for milling, turning or hobbing operations. As such, the previously mentioned methods have failed to provide a sufficient treatment of the cutting edge. Hand treating is inconsistent. Brushing, by the method discussed above, breaks carbide particles out of the cutting edge rather than rounding it properly. Sand blasting produces an undefined radiused cutting edge in conjunction with roughening the surface of the cutting side profile surface and the front face. Drum rotation with a particulate abrasive is not suitable due to the weight and dimension of stick-type cutting blades, particularly cutting blades made of carbide material.
It is an object of the present invention to provide a method of rounding the cutting edge of a cutting blade for producing toothed articles that substantially reduces or eliminates breakage of particles out of the cutting edge.